The exhaust passage of a diesel engine is sometimes fitted with a lean NOx catalytic converter (referred to as LNC hereinafter) for reducing and eliminating nitrogen oxides (referred to as NOx hereinafter) from the exhaust gas. In an LNC, NOx is absorbed when the air fuel ratio of the exhaust gas (referred to exhaust A/F hereinafter) is higher than a prescribed level (referred to as a lean condition hereinafter) or when the oxygen content of the exhaust gas is high, and the absorbed NOx is released and reduced to a harmless form when the exhaust A/F is higher than the prescribed level (referred to as a rich condition hereinafter) or when the oxygen content of the exhaust gas is low. Because the capability of the LNC to absorb NOx diminishes as the build up of the NOx therein increases, the combustion state is varied so that the concentration of the reducing agents such as CO and HC is increased while the oxygen concentration is decreased. Thereby, the release of NOx from the LNC is enhanced, and the reduction of the NOx is promoted.
Because the fuel includes sulfur contents, sulfur oxides (referred to as SOx hereinafter) and hydrogen sulfide (referred to as H2S hereinafter) are also emitted with the exhaust gas. When such sulfur contents are absorbed by the LNC (referred to as sulfur poisoning hereinafter), the capability of the LNC to absorb NOx diminishes. Therefore, it is necessary to release the sulfur contents absorbed in the LNC from time to time. The process of releasing sulfur contents from a LNC (referred to as sulfur purging hereinafter) requires that the temperature of the LNC and air fuel ratio of the exhaust gas are both at appropriate levels. It is known that it can be achieved by carrying out a secondary fuel injection following the combustion (referred to as post injection hereinafter) in addition to the main fuel injection that is carried out during the intake stroke of the engine to increase the temperature of the LNC and injecting a prescribed amount of fuel serving as a reducing agent into the exhaust gas under a feedback control based on the output of the air fuel ratio sensor to maintain the exhaust air fuel ratio at a prescribed rich level (see Japanese patent laid open publication Number 2003-120373).
It is also known to conduct a post fuel injection to control the exhaust air fuel ratio in such a manner as to execute a favorable sulfur purging (see Japanese patent laid open publication Number 2004-204812).
However, as the method described in Japanese patent laid open publication Number 2003-120373 requires that the catalyst temperature increase control by a post fuel injection and the enriching control for the exhaust air fuel ratio by supplying a reducing agent are required to be executed individually, the control structure is complex, and the need for an injection valve in the exhaust passage for the supply of the reducing agent complicates the engine structure.
According to the technology described in Patent Document #2, because the control target value of the exhaust air fuel ratio is fixed in advance, the control action may not be necessarily suitable for each particular condition of the LNC, and the sulfur purge cannot be executed in an optimum fashion under all conditions.